Signalling and Safety at Sea, 27 



bearings as north and south and the danger rate as one 

 knot. 



As we have already seen, the knowledge of the danger 

 bearing may rule out the possibility o£ collision. For it! the 

 actual bearing is decisively different from the danger bearing 

 there cannot be collision. By radio-goniometer — an instru- 

 ment we must refer to again later on — the bearing may be 

 found, at least approximately. Or by submarine sounds an 

 approximate bearing may be taken. If the Fessenden 

 Oscillator is carried on the ships the bearing could be deter- 

 7nined while yet 10 or more miles separated the vessels. If 

 the bell is carried the bearing could be found some 6 or 7 

 miles away under normal conditions. The determination of 

 the bearing will, probably, be the first procedure. But we 

 will suppose that the bearing as observed — however 

 determined — is doubtful ; that it cannot be safely dis- 

 criminated from the danger bearing. And as the sailor 

 knows not with any certainty at this stage the distance 

 separating the vessels, he regards it as unsafe to undertake 

 prolonged observations of the bearing, and decides on finding 

 the distance and the rate of approach. 



When he determines on this he makes a code signal 

 announcing to the other ship that he is about to find the 

 distance by synchronous signals. If this is announced by A, 

 B prepares to listen. When at length B picks up the 

 oscillator or bell she tells A the distance as so many miles. 

 This first observation of distance assures to the sailor a 

 knowledge of the time at his disposal. For suppose — 

 reverting to our example — that B says "our distance is 

 5 miles." Then both on A and B it is known that collision 

 cannot occur sooner than 12 minutes from that instant. 

 For on A and B it is already known that the danger rate or 

 maximum rate of approach for the courses and speeds is 0*8 

 knot in 2 minutes, or 0'4 knot in 1 minute. Hence we have 

 to divide 5 by 0*4 to get the interval in minutes before 

 collision can occur ; and this gives 12 \ minutes. In this 

 way the sailor knows at the earliest moment at which the 

 submarine signals are audible from ship to ship how much 

 time is available for further observation. 



Two minutes after the first observation of distance, a 

 second synchronous signal is sent out — say from A — and B 

 says "our distance is 4 miles." This looks like danger. 

 For there is some error certainly seeing that the approach 

 oannot be so much as 1 mile in 2 minutes. There is now 10 

 minutes to go and there is no reason why several more 

 distance determinations should not be made. The emission 



